The performance of modern-day military helicopters, missiles, tanks, aircraft and other static or dynamic structures is critically dependent on the reliability of advanced composite materials and heterogeneous armor materials. There has been a reluctance to deploy such high performance materials in critical structural applications because of their susceptibility to in-service damage. The damage occurring in these materials may be difficult to track and can propagate quickly during operation of the vehicle or structure, resulting in the loss of the entire vehicle.
Conventional non-destructive evaluation techniques are labor intensive, expensive, error prone, and unworkable for efficient integration into composite and heterogeneous structures. Autonomous integrated Structural Health Monitoring (SHM) techniques are a revolutionary concept in the maintenance of structures. SHM techniques continuously monitor the condition of a structure. Various approaches for SHM under development use piezoceramic (Lead zirconate titanate (Pb[ZrxTi1-x]O3 0≦x≦1), also called PZT,) sensors and actuators that require separate wiring connections for each sensor and actuator element, storage of pre-damage data for each sensor, and instrumentation for active generation and sensing of diagnostic signals. When the structural geometry is complex—e.g., either the structure has varying thickness, curvature, ribs, joints or heterogeneous materials, or damage is located near boundaries of the structure—it becomes difficult to detect small damage using SHM methods. In addition, the number of sensor circuits and computations required increases the overall complexity and cost of the structure.
One approach to this problem is to integrate many fiber-optic strain gauges directly within the structural material. An optical fiber with twenty or more Bragg gratings can measure static and dynamic strains at discrete locations on the structure. An optical analyzer can multiplex over each fiber and each grating to measure strains at a large number of points on a structure. This approach is being implemented on bridges, pressure tanks and other structures. However, fiber optic sensors have limitations when applied to monitoring complex composite structures where damage can occur anywhere on the structure and in any direction. For example, discrete strain measurements can miss damage because the measurement is very localized at the fiber/grating. In addition, an optical analyzer using multiplexing and multiple connections is expensive; measurements are not simultaneous and the frequency bandwidth may be too low to sense Acoustic Emission (AE) signals.
AE sensors are presently suitable for detection of damage at “hot spots.” The use of AE measurements for SHM of large structures may have certain advantages since it is a passive sensing technique. Passive sensing methods are simpler and may be more practical than using active interrogation methods. However, present passive acoustic emission and monitoring techniques require bulky instrumentation with numerous channels, long connections, and centralized data analysis. It may be impractical to embed these systems on the structure to operate in the field. Another limitation is that AE waveforms from such sensors are too complicated for purposes of source characterization.
U.S. Pat. No. 6,399,939 issued Jun. 4, 2002 to Sudaresan et al. discloses a sensor array wherein the number of sensors and instrumentation channels required was reduced, by an order of magnitude, while retaining the sensitivity in the high frequency range to detect incipient damage in the structure. The disclosure of this patent and its cited references is hereby incorporated by reference in its entirety.
U.S. Pat. No. 7,075,424 issued Jul. 11, 2006 to Sudaresan et al. discloses a sensor array wherein only one channel of AE instrumentation is required for locating the AE source within a region since the output on a timed scale is used to calculate the location of the critical structural event. The disclosure of this patent and its cited references is hereby incorporated by reference in its entirety.
U.S. patent application Ser. No. 11/271,156, filed Nov. 10, 2005 to Sudaresan et al. discloses sensor assemblies for non-destructively monitoring a structure to detect a structural event. The disclosure of this patent application and its cited references is hereby incorporated by reference in its entirety.
U.S. Provisional Patent Application Ser. No. 61/449,935 filed Mar. 7, 2011 to Sudaresan discloses techniques for identifying source type and the presence of the shear wave, the contents of which are hereby incorporated herein by reference in its entirety.
Thus, there remains a need for a new and improved system for non-destructively monitoring a structure to detect a critical structural event using a sensor array including acoustic emission sensors while, at the same time, includes shear wave sensors which are adapted to prevent false positives of critical structural events.